In the visual apparatus field, in order to improve image sharpness, contour correction circuits for correcting video signals have been used so as to emphasize contours of the picture signals.
Conventionally used contour correction circuits generate a contour correction signal from high-frequency components which may be extracted from an input video signal, by passing it through a high-pass filter, by a differentiation process or the like, and superimpose the extracted contour correction signal over the delayed input video signal by an adder, to thereby produce a video signal enhanced in sharpness at contours.
However, since the above usually used, conventional contour correction method performs contour correction by emphasizing the supplied reproduction luminance signal over the range from low-luminance to high-luminance in the same manner, in some cases noise existing at areas where the luminance signal is low may also be enhanced; in other cases while excessive overshoots or undershoots of large amplitudes may occur around rising edges at which the luminance signal sharply rises from black to white levels or around falling edges at which the luminance signal sharply drops from white to black levels, whereby boundaries between black and white areas are unnaturally emphasized producing contour blurs or noise is also emphasized at the same time, resulting in degradation of image quality.
In order to solve the above problems, a method has been disclosed in Japanese Patent Application Laid-open Hei 7 No. 7636.
The system disclosed in Japanese Patent Application Laid-open Hei 7 No. 7636 includes a correcting means for performing contour correction of emphasizing the levels of all the tones of the luminance signal in like manner; a halftone emphasizing means for emphasizing only the levels corresponding to halftones of the luminance signal; and a multiplying means for multiplying the luminance signal, which has been contour corrected by, and is output from, the correcting means, by the halftone emphasized signal, or the signal which has been emphasized as to the levels corresponding halftones only and is output from the halftone emphasizing means, and is characterized in that only the contours with their luminance signal belonging to medium levels are corrected.
Since this method emphasizes the medium tone levels only, without emphasizing noises that exist in the areas where the luminance signal level is low, it is possible to obtain high quality images free from conspicuous noises. Further, since no level emphasis will be achieved for rising edges at which the luminance signal sharply changes from black to white levels or for falling edges at which the luminance signal sharply drops from white to black levels, no excessive overshoots or undershoots of large amplitudes will occur. As a result, contours at the boundaries between black and white areas will be never emphasized unnaturally, so this method can be thought as an invention which is able to make such boundary areas markedly clear.
However, this method uses filtration along the horizontal or vertical direction, hence suffers from the problem of oblique lines becoming jagged because the filtration disrupts the gradation of oblique lines etc., in oblique directions.
In view of the above problems, the present invention is to provide a contour correcting device free from the above problems.